Cobalt-based tripodal complexes as molecular catalysts for photocatalytic CO2 reduction.

Construction of artificial photosynthetic systems including CO2 reduction is a promising pathway to produce carbon-neutral fuels and mitigate the greenhouse effect concurrently. However, the exploitation of earth-abundant catalysts for photocatalytic CO2 reduction remains a fundamental challenge, which can be assisted by a systematic summary focusing on a specific catalyst family. Cobalt-based complexes featuring tripodal ligands should merit more insightful discussion and summarization, as they are one of the most examined catalyst families for CO2 photoreduction. In this feature article, the key developments of cobalt-based tripodal complexes as molecular catalysts for light-driven CO2 reduction are discussed to offer an upcoming perspective, analyzing the present progress in electronic/steric tuning through ligand modification and dinuclear design to achieve a synergistic effect, as well as the bottlenecks for further development.

Ru Monoimines with Extended Excited-State Lifetimes and Geometrical Modulation of Photoinduced Mixed-Valence Interactions.

The photophysical properties of monodentate-imine ruthenium complexes do not usually fulfil the requirements for supramolecular solar energy conversion schemes. Their short excited-state lifetimes, like the 5.2 ps metal-to-ligand charge transfer (MLCT) lifetime of [Ru(py)4Cl(L)]+ with L = pz (pyrazine), preclude bimolecular or long-range photoinduced energy or electron transfer reactions. Here, we explore two strategies to extend the excited-state lifetime, based on the chemical modification of the distal N atom of pyrazine. On one hand, we used L = pzH+, where protonation stabilized MLCT states, rendering thermal population of MC states less favorable. On the other hand, we prepared a symmetric bimetallic arrangement in which L = {(µ-pz)Ru(py)4Cl} to enable hole delocalization via photoinduced mixed-valence interactions. A lifetime extension of 2 orders of magnitude is accomplished, with charge transfer excited states living 580 ps and 1.6 ns, respectively, reaching compatibility with bimolecular or long-range photoinduced reactivity. These results are similar to those obtained with Ru pentaammine analogues, suggesting that the strategy employed is of general applicability. In this context, the photoinduced mixed-valence properties of the charge transfer excited states are analyzed and compared with those of different analogues of the Creutz-Taube ion, demonstrating a geometrical modulation of the photoinduced mixed-valence properties.

Solvent influence on non-adiabatic interfacial electron transfer at conductive oxide electrolyte interfaces.

The kinetics for interfacial electron transfer (ET) from a transparent conductive oxide (tin-doped indium oxide, ITO, Sn:In2O3) to molecular acceptors 4-[N,N-di(p-tolyl)amino]benzylphosphonic acid, TPA, and [RuII(bpy)2(4,4'-(PO3H2)2-bpy)]2+, RuP, positioned at variable distances within and beyond the electric double layer (EDL), were quantified in benzonitrile and methanol by nanosecond absorption spectroscopy as a function of the thermodynamic driving force, -ΔG°. Relevant ET parameters such as the rate constant, ket, reorganization energy, λ, and electronic coupling, Hab, were extracted from the kinetic data. Overall, ket increased as the distance between the molecular acceptor and the conductor decreased. For redox active molecules within the Helmholtz planes of the EDL, ket was nearly independent of -ΔG°, consistent with a negligibly small λ value. Rips-Jortner analysis revealed a non-adiabatic electron transfer mechanism consistent with Hab < 1 cm-1. The data indicate that the barrier for electron transfer is greatly diminished at the conductor-electrolyte interface.

Wave-Function Symmetry Control of Electron-Transfer Pathways within a Charge-Transfer Chromophore.

Despite a diverse manifold of excited states available, it is generally accepted that the photoinduced reactivity of charge-transfer chromophores involves only the lowest-energy excited state. Shining a visible-light laser pulse on an aqueous solution of the chromophore-quencher [Ru(tpy)(bpy)(µNC)OsIII(CN)5]- assembly (tpy = 2,2';6,2''-terpyridine and bpy = 2,2'-bipyridine), we prepared a mixture of two charge-transfer excited states with different wave-function symmetry. We were able to follow, in real time, how these states undergo separate electron-transfer reaction pathways. As a consequence, their lifetimes differ in 3 orders of magnitude. Implicit are energy barriers high enough to prevent internal conversion within early excited-state populations, shaping isolated electron-transfer channels in the excited-state potential energy surface. This is relevant not only for supramolecular donor/acceptor chemistry with restricted donor/acceptor relative orientations. These energy barriers provide a means to avoid chemical potential dissipation upon light absorption in any molecular energy conversion scheme, and our observations invite to explore wave-function symmetry-based strategies to engineer these barriers.

Kinetic Evidence That the Solvent Barrier for Electron Transfer Is Absent in the Electric Double Layer.

Classical capacitance studies have revealed that the first layer of water present at an aqueous metal-electrolyte interface has a dielectric constant less than 1/10th of that of bulk water. Modern theory indicates that the barrier for electron transfer will decrease substantially in this layer; yet, this important prediction has not been tested experimentally. Here, we report the interfacial electron transfer kinetics for molecules positioned at variable distances within the electric double layer of a transparent conductive oxide as a function of the Gibbs free energy change. The data indicate that the solvent reorganization is indeed near zero and increases to bulk values only when the molecules are positioned greater than 15 Å from the conductive electrode. Consistent with this conclusion, lateral intermolecular electron transfer, parallel to a semiconducting oxide electrode, was shown to be more rapid when the molecules were within the electric double layer. The results provide much needed feedback for theoretical studies and also indicate a huge kinetic advantage for aqueous electron transfer and redox catalysis that takes place proximate to a solid interface.

A Hole Delocalization Strategy: Photoinduced Mixed-Valence MLCT States Featuring Extended Lifetimes.

Bimetallic trans-[RuII(tpm)(bpy)(µNC)RuII(L)4(CN)]2+, where bpy is 2,2'-bipyridine, tpm is tris(1-pyrazolyl)methane and L = 4-methoxypyridine (MeOpy) or pyridine (py), was examined using ultrafast vis-NIR transient absorption spectroscopy. Of great relevance are the longest-lived excited states in the form of strongly coupled photoinduced mixed-valence systems, which exhibit intense photoinduced absorptions in the NIR and are freely tunable by the judicious choice of the coordination spheres of the metallic ions. Using the latter strategy, we succeeded in tailoring the excited state lifetimes of bimetallic complexes and, in turn, achieving significantly longer values relative to related monometallic complexes. Notable is the success in extending the lifetimes, when considering the higher density of vibrational states, as they are expected to facilitate nonradiative ground-state recovery.

Inversion of donor-acceptor roles in photoinduced intervalence charge transfers.

Upon MLCT photoexcitation, {(tpy)Ru} becomes the electron acceptor in the mixed valence {(tpy˙-)RuIII-δ-NC-MII+δ} moiety, reversing its role as the electron donor in the ground-state mixed valence analogue. Photoinduced mixed valence interactions can be tuned to obtain extended lifetimes and higher emission quantum yields, beneficial in supramolecular energy conversion schemes.

Influence of the electronic configuration in the properties of d6-d5 mixed-valence complexes.

We report here the spectroscopic properties of four very closely related mixed-valence cyanide-bridged bimetallic complexes, trans-[Ru(T)(bpy)(µ-NC)Ru(L)4(CN)](3+) (T = tris(1-pyrazolyl)methane (tpm, a) or 2,2';6',2″-terpyridine, (tpy, b), and L = pyridine (py, 1) or 4-methoxypyridine (MeOpy, 2)). In acetonitrile all the complexes present intervalence charge transfer (IVCT) transitions in the NIR region, but their intensities are widely different, with the intensity of the transition observed for 1a-b(3+) around four times larger than that observed for 2a-b(3+). This contrasting behavior can be traced to the different nature of the dπ acceptor orbitals involved in these transitions, as confirmed by (TD)DFT calculations. The spectroscopy of 1a-b(3+) provides evidence that the spin density is delocalized over the two ruthenium ions, such as a narrowing of the IVCT bands that results in the resolution of the expected three bands, and a weak solvent dependence of the energy of these transitions. The spectroscopy of 2a-b(3+) instead indicates that the spin density is localized on one ruthenium ion. The IVCT in these systems is particularly weak due to the configuration of the Ru(III), where the vacant orbital is perpendicular to the cyanide bridge.

Emissive cyanide-bridged bimetallic compounds as building blocks for polymeric antennae.

A series of cyanide-bridged bimetallic compounds of the general formula [Ru(L)(bpy)(µ-NC)(M)](2-/-/2+) (L = tpy, 2,2'-6',2''-terpyridine, or tpm, tris(1-pyrazolyl)methane, bpy = 2,2'-bipyridine, M = Ru(II)(CN)5, Os(III)(CN)5, Os(II)(CN)5, Ru(II)(py)4(CN), py = pyridine) have been synthesized and fully characterized. Most of them present MLCT emission (λ = 690-730 nm, Φ = 10(-3)-10(-4)) and their photophysical properties resemble the ones of the respective mononuclear Ru(L)(bpy) species. The exception is when M is Os(III)(CN)5, where an intramolecular electron transfer quenching mechanism is proposed. The conditions that should be met for avoiding the reductive or oxidative quenching of the excited state are also discussed.